This invention relates to electrical interconnections, and to methods and apparatuses for forming such interconnections. More particularly, the invention is directed to a method and apparatus for attaching electronic components, especially chips or chip carrier packages, to supporting substrates, such as circuit boards.
The microelectronics industry is steadily moving toward the use of large chips and chip carrier packages (CCP) which have connection pads on the faces and/or edges. These are generally used where there are limitations with the use of dual inline packages (DIP). The number of connections on the most popular packages can range from 64 to 156. Chip carrier packages can be produced with leads attached (leaded) or they can be leadless.
Leaded CCPs can be soldered directly onto printed circuit boards (PCB) or printed wire boards (PWB). Leadless CCPs can be soldered onto ceramic boards or installed into connectors. However, with glass/epoxy printed circuit boards or printed wire boards, leadless CCPs are usually mounted into connectors which are in turn mounted on the PCBs because of the effect of the differential thermal expansion coefficients of the materials involved when subjected to temperature fluctuations. These connectors are complex to manufacture and costly to use.
As the CCP technology improves and their reliability increases, more emphasis is directed to soldering these packages directly onto the PCBs to make more use of the board space, and dispensing with the connectors even though the use of connectors permits replacement of faulty CCPs. The cost of conventional connectors relative to the cost of the CCPs can be disproportionately high. This is a strong incentive to use CCPs without connectors.
However, the direct soldering of the CCPs on PCBs without the use of connectors is associated with a number of problems. (1) The variation of the surface flatness and non-parallel contours between the CCP and the boards produces varying solder joint heights. (2) The solder will have a tendency to wick out of the joint area into crevices or castellations in the CCP, thus "starving" the joint area. (3) Gold alloying with the solder will produce embrittlement of the "starved" joint. (4) Differential thermal expansion between the CCP and the board will fracture a thin solder joint due to the high shear strains in the joint. (5) Bridging between joints may occur if excess solder is present or if the distance between the joints is small. (6) Solder location tolerances are small and getting smaller yet with increasing packaging density, with a typical connection pad having a width of 0.012 inches and a 0.022-inch center-to-center spacing. (7) Flux removal from the space between the CCP and the PCB and subsequent inspection thereof. (8) The solder pastes used to attach the CCP will produce loose solder balls which cause electrical problems.
Problems (1) through (4) and problems (7) and (8) can be substantially overcome by introducing tall columns of solder or similar joint-forming material to form each joint. This provides a "solder-rich" condition which minimizes the gold alloying effect, provides excess solder to accommodate surface irregularities and non-flatness, the tall solder columns will have lower shear, bending and/or combined stress and strain proportional to their greatest height, and no solder balls will be created. Problems (5) and (6) require the precise control of solder quantity and location.
Different solutions have been proposed for the foregoing problems. The proper positioning of a predetermined quantity of solder may be achieved with the use of solder preforms spaced on a carrier or template in the locations corresponding to the points where the solder joint are to be formed. Examples of this technique may be found in U.S. Pat. Nos. 3,320,658, issued to Bolda et al; 3,396,894, issued to Ellis; 3,472,365, issued to Tiedema; 3,719,981, issued to Steitz; 3,744,129, issued to Dewey; 4,209,893, issued to Dyce et al.; and 4,216,350, issued to Reid.
Dyce et al. and Reid relate more specifically to the use of ring-type solder preforms for the solder connection of pin-type joints. The solder preforms of Dewey are hollow cylinders. Tiedema relates to a flexible carrier ribbon having spaced apertures which receive solder discs to provide a convenient means to handle and transport solder discs.
The Ellis patent discloses a device for simultaneously applying a plurality of solder or other bodies of heat-fusible material, in which the solder bodies are disposed in heat-recoverable cups formed from or positioned on a sheet of material. The cups are spaced to correspond to the location of solder application, and, when heat is applied to the device, the solder melts and the cups recover to a flat configuration, and the recovering cup material forces the solder material out therefrom and into contact with the elements to be soldered.
The Bolda et al. patent provides a thermoplastic carrier sheet onto which a plurality of conductive elements, such as solder preform spheres, are positioned. The individual conductive elements are heated to a temperature which is sufficient to soften the carrier material, but insufficient to deform the conductive elements. During the heating process, the conductive element nestles in the softened thermoplastic material and, upon removal of the heat, the carrier material resolidifies and rigidly supports the conductive elements. When the solder ball carrier assembly is used, heat is applied to melt the solder ball and soften the thermoplastic carrier material, permitting the material to form an insulation between those portions of the electrical conductors not electrically interconnected by the solder elements.
Another approach has been proposed by Bell Labortories which is developing techniques employing vacuum equipment to pick up and place small solder balls on the underside of the CCPs, and retaining them by using a solid phase bonding method. Additional information may be found in the paper by R. H. Minetti entitled "Solid Phase Solder Bonding for Use in the Assembly of Microelectronic Circuits."
Steitz provides a method of joining solder balls to solder bumps spaced on a semiconductor flip chip by forming an array of solder balls on the tacky side of a pressure-sensitive tape, with the balls being spaced like the solder bumps. The array of solder balls is placed in contact with the solder bumps, and both are then heated to reflow the solder and cooled to fix the contacts after which the tape is removed.
In U.S. Pat. No. 3,614,832, Chance et al. form connections between the contacts on a solid-state device and conductive lands on a substrate by placing a decal over the device, the decal having a plurality of conductive strips attached adhesively to a backing sheet. Application of heat and pressure effects bonding of the strips, after which the adhesive is dissolved and the decal removed.
Although the foregoing techniques provide for the correct placement of a predetermined quantity of solder or other suitable joint-forming material, and with the proper dimensioning of the carrier or template, sufficiently small quantities of solder can be positioned on close spacing between centers, these proposals do not address the problem of high shear strains in the solder joints.
As noted above, among the factors considered in forming acceptable electrical connections between the CCP and the PCB is that the connections must be able to withstand stresses developed due to the effect of temperature fluctuations and the differences in thermal expansion coefficients between the material of the CCP and the substrate or circuit board on which it is mounted. Thus, a CCP may be made of a ceramic material and the circuit board may be made of an epoxy-glass composition, and when subjected to elevated temperatures these elements will expand at different rates, inducing stresses in the connections.
Even if the materials used in the CCP and the circuit board have thermal expansion coefficients which are close in value to minimize the differential expansion effects, heating/cooling cycles which result when power is applied across the CCP induce a temperature differential between the CCP and the PCB to produce stresses in the joints.
It is well known, and as summarized below, that if the solder joint is formed into a "long" column configuration in which the height of the column is much greater than the diameter or transverse dimension of the joint, less stress is induced in the joint and consequently the joint has greater reliability and longer life.
A common deficiency of the solder connections described in the foregoing references is that the height of the joint is smaller than its width or diameter, and the height is significantly smaller than the center-to-center distance between consecutive joints. This is true since the joints are formed from free solder preforms in a variety of shapes. None of these preforms can sustain a column-type shape after melting and during the soldering process due to the low strength and viscosity and high surface tension of molten solder. These joints contrast with the present invention in which the height of the resultant joints is greater than the transverse dimension of the solder joints.
In the patent to Krall, U.S. Pat. No. 3,921,285, a method is described for joining microminiature components to a carrying structure in which the height of the electrical connections may be adjusted during original joining of the component to the carrying structure or in a two-step solder reflow process. The method involves the elongation of the solder joints between the component and its carrier, and is accomplished by the use of a vaporizable material which is either liquid at room temperature or becomes liquid before the solder melts. A bridge is positioned over the component and the vaporizable material is placed between the bridge and the component surface opposite the surface on which the connections are formed. Heating to achieve soldering causes the material to vaporize, and the combined action of vaporization and surface tension pulls the component closer to the bridge, which in turn elongates the solder joint. Upon cooling the joint remains fixed in its elongated shape.
While Krall provides for elongated solder joints, the device is structurally complex and difficult to use. Specifically, an additional lifting structure is required to operate while the solder is in a molten state. If the motion of the lifting device is too great, or if the solder quantities are not uniform, then some solder joints may be ruptured. Additionally, the lifting structure is an obstacle to cleaning and inspection. Finally, this method will function only when the ratio of solder column height to diameter is relatively small.